This invention relates to making and using sintered metal copings, bridges, onlays, inlays, crowns, and the like which have superor physical properties and which show superior dimensional fit, separation, and alignment than the usual cast metal prosthetic devices. Other short run, biomechanical devices may also be constructed by these same techniques.
Hitherto metallic dental prosthetic devices have been cast by processes such as investment casting, the "lost wax" process, to yield a homogeneous microstructure of one fine phase or multiphase fine grain microstructures with appreciable porosity. The microstructure of the porosity of cast metals is irregular and may even show channels. Often the porosity is spherical in the general range of one micron. The level of porosity is about one percent.
The liquid phase sintered dental devices of the present invention exhibit under an optical microscope a composite structure. In this novel structure discontinuous grains are dispersed in a continuous matrix. Even if the starting material of the sintered alloy is of uniform composition, the heterogeneous two-phase product may show higher melting grains of one composition in a lower melting matrix of another composition. This disparity in composition arises from the fact that the sintering is carried out at a temperature between that of the liquidus and solidus temperatures.
Making and using the sintered powdered alloy devices, having multiple units of this invention, obviates the soldering together of single units, which leads to porosity and inferior alignment, and soldering difficulties caused hitherto by oxidation of the metallic surfaces.
Cast, full-coverage dental restorations inherently result in unsatisfactory seating. There are so many steps to the casting process that multiple units are not well aligned because of the many opportunities for dimensional errors. Thus in the classical casting of dental devices the margins of the casting do not generally coincide with the finish line of the prepared tooth. This is especially so for castings of non-precious metals, because there is excessive contraction of the alloy as it cools from the molten state to the solid state. The remedies are to grind away the interferences in the newly cast restoration and grind the opposing natural tooth to permit the jaws of the patient to close normally. Or when working the cast bridges, the dental technician employs multiple layers of spacer materials on the dies of the prepared teeth resulting in a very loose fit conpensated for by excessive cement.
Until this invention practitioners did not match the physical expansion, the thermal expansion, and variations in both of the die materials with the thermal expansion of the sintering alloy to achieve a fit in dental devices at room temperature consistently less than 50 microns.
Furthermore, until this invention no one has used two different die materials: an abutment die material to make a preform for the copings or crowns and a base die material to join the various segments of the bridge, so that a fit of less than 50 microns results when multitooth dental devices are made from die materials with different physical and thermal expansions.
The conventional method for making a dental preform relies on the "lost wax" technique, which involves casting molten metal into a hollow ceramic investment material shaped around an original wax model.
The wax model is formed to fit precisely a die which is a replica of the prepared surface of the teeth made by the dentist from a dental impression. Therefore, to make a cast alloy or porcelain dental device, such as a full cast crown, veneer crown, coping, pontic, inlay, onlay, or bridge by the lost wax process involves six steps between seven entities: prepared tooth, impression, die, wax model, hollow investment, metal replica, dental device. Trying to accomplish this six-stage process with good dental "fit" leads to variable results ranging from +200 microns or more to a negative fit which cannot be seated upon the original die or prepared tooth.
In general, the following steps are customary in the lost wax process:
(1) The dentist prepares a tooth or teeth to be restored in different ways according to which prosthodontic device is to be used: crown, bridge, splint, or fixed partial denture.
(2) The dentist then makes an impression of the prepared tooth or teeth in an accurate manner. From this impression a model is made. With this model or replica the dentist supplies an accurate duplication of the patient's opposing arch and a bite registration in what is known as centric relation. Thus a highly accurate duplication of the prepared teeth on which the prosthesis is to be fabricated and the maxillo-mandibular relationship is provided in model form. Even in the restoration of a single crown, the dentist provides the technician with an accurate duplication of the adjacent teeth, as well as the opposing teeth, in order to permit the building of contact points and occluding contact point in both the rest position of the jaws as well as in masticatory movement.
(3) Since the final restoration of a crown or bridge must harmonize with the patient's dentition in appearance as well as function, a precise model permits fabrication of a wax pattern comforming to a specific design for a dental device.
(4) The finished wax pattern is sprued, then removed from the die, and connected to a sprue former using a precise system of waxes to attempt later complete casting of the metal. Since the wax pattern is removed from the die to be invested, this is considered an indirect fabrication technique. It is important to note that discrepancies may be introduced in an indirect technique due to distortion of the wax pattern during its removal from the die. The wax may distort in the investing process. The wax and investment materials may undergo contraction or expansion due to temperature changes during setting and burn-out of the investment and casting, and solidification of the molten metal during the cooling cycle.
(5) The sprued wax pattern is then invested in a high heat material, depending upon the type of metal being cast.
(6) The invested wax pattern, when set (cured), is placed in a burn-out oven for a period of one and a half hours or more, depending upon the technique and metal being cast. Temperatures in the burn-out oven may range from about 650.degree. C. to about 1000.degree. C., in one or more heat stages to insure maximum expansion of the investment. This expansion during the heating cycle varies and may be a cause of an improper fit of the final restoration, if not properly controlled. The burn-out procedure not only expands the investment in preparation for the casting of the molten metal, but is essential for elimination of all the wax, thus leaving a void in the investment material that is a mould of the eliminated wax pattern.
(7) A casting ring is employed to contain the investment material around the wax pattern. Spacers in the casting ring are used to permit expansion of the investment during the heating stage. This may lead to an imprecise fit of the final casting. Some investment materials employ a plastic or paper ring for forming and containing the investment material. The plastic or paper is either burned-off during the burn-out state or removed after the investment has set. This allows for maximum expansion of the investment during the heating stage. The actual casting is made by placing the investment, which was formed by some kind of device, into some type of castng apparatus after the burn-out stage. This permits melting of the desired metal at the required temperature. The molten metal is then forced into the mould in the hot investment either by centrifugal force, pressure, or vacuum. There are different types of equipment for these various methods of making a casting. Once the cast has been completed, the metal and investment material are allowed to cool.
(8) The casting must be recovered from the investment material by breaking it out from the investment. The casting is then cleaned off.
(9) The sprues are then cut off the crown, bridge or pontic and smoothed down. The casting must be then fitted back upon the original die. If the technician utilizes a gypsum die, it may be difficult to seat the casting on the model without scraping or chipping the die. The ultimate fit on the tooth is therefore complicated for the dentist.
Miscasts and incomplete casting which fail to reproduce fine details of the original wax pattern, or poor fit of the casting due to contraction and expansion of the wax and investment material are not uncommon. This may require repeating the entire procedure, if the casting cannot be properly seated on the die or the prepared tooth of the patient.
While dental bridges can either be cast in one piece or assembled from individual units, a more accurate fit is assured by assembly of the units of the bridge or splint from an index impression taken of the units seated in the mouth. This insures complete placement of the device made from the castings upon the individually prepared teeth. This technique has been widely employed for many years, utilizing precious metal alloys which are relatively simple to solder. The non-precious metal alloys employed today are more difficult to solder or braze due to formation of oxide layers on their surfaces when subjected to high temperatures. This had led the dentist to prescribe casting multiple unit bridges and splints in one piece to eliminate the necessity for soldering. There is much question regarding accuracy of fit of such long-span prostheses cast as one device.
These long, labor-intensive techniques of the prior art are costly as well as time-consuming, and often provide a questionable or inaccurate fit.
In the patent literature various ceramic, metallic, and ceramo-metallic materials have been employed in attempts to improve crowns and bridges. Such materials are disclosed in:
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